KVM: s390: Provide access to program parameter

commit d208c79d

 (KVM: s390: Enable
the LPP facility for guests) enabled the LPP instruction for guests.
We should expose the program parameter as a pseudo register for
migration/reset etc. Lets also reset this value on initial CPU
reset.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>